This invention relates to a gas chromatography system and methods for increasing the speed, operational flexibility, and accuracy of gas chromatography procedures.
Gas chromatography is a widely employed technique for the separation and analysis of complex mixtures of volatile organic and inorganic compounds. The mixture is separated into its components by eluting them from a column having a sorbent by means of a moving gas.
Gas chromatography procedures can be classified into two major divisions: gas-liquid chromatography and gas-solid chromatography. Gas-liquid chromatography is presently the most widely employed type and incorporates a nonvolatile liquid sorbent coated as a thin layer on an inert support structure, generally a capillary tube. The moving gas phase called the carrier gas flows through the chromatographic analytical column. The analyte partitions or divides itself between the moving gas phase and the sorbent and moves through the column at a rate dependent upon the partition coefficient or solubility of the analyte components. Various types of analytical columns are employed such as tubular glass or stainless steel capillary tubes. In use, the analyte is introduced at the entrance end of the column within the moving carrier gas stream. The components making up the sample become separated along the column and elute from the exit end of the column at intervals and in concentrations characteristic of the properties of the analyte components. A detector, for example, a thermal conductivity detector or a flame ionization detector (FID) at the exit end of the column responds to the presence of analyte components. Upon combustion of the eluted material in the FID, charged species are formed in the flame. The flame behavior is monitored through a biased ion detector which, along with associated electronics, produces a time versus magnitude trace of the detector output. The trace for a complex mixture includes numerous peaks of varying intensity. Because individual constituents of the analyte produce peaks at characteristic times having magnitudes that are a function of the constituent concentration, much information is gained through an evaluation of the chromatogram.
Various approaches are presently used for introducing a sample into the separation column. In one general type of gas chromatography system, a thermal focusing chamber or cold trap is employed. The cold trap is typically a vessel containing a cold gas such as nitrogen and having a capillary sample tube passing through it which conducts the analyte. By exposing incoming analyte to the low temperatures within a cold trap, the analyte components condense on the capillary tube. When it is desired to inject a sample into the column for separation, the temperature of the sample tube passing through the cold trap is increased rapidly thus vaporizing the sample. The carrier gas stream which continually flows through the trap then injects the analyte into the column for separation.
In a typical gas chromatography system of the type employing a thermal focusing chamber, during the trapping mode of operation, a single cold trap having a capillary tube at the inlet end of the cold trap sample tube (i.e., in the direction of carrier gas flow during injection) traps the incoming analyte. After heating the cold trap sample tube, the sample components must traverse the entire length of the sample tube before introduction into the column. The sample flow circuit regions between where the component is vaporized and the beginning of the column constitute system "dead volume" which is undesirable because it results in broadening of the injected analyte in terms of the time duration over which it is presented to the inlet end of the column. Dead volume adversely affects system resolution and efficiency.
Today there is increased emphasis toward so called "high-speed gas chromatography" or "high speed GC". Applications include process stream monitoring, environmental monitoring, and engine exhaust gas analysis. Ideally such systems would be able to perform an analysis within several seconds which previously took several minutes or more. Increasing the speed of analysis can be achieved by providing a relatively short separation column or by using other techniques for causing components of interest to traverse the column quickly. In order to provide useful information, the individual analyte components must elute separately at the detector, thus producing distinct peaks. As the length of time that the sample is injected at the inlet end of the column increases, the peaks produced by elution of the components tend to broaden. It is, therefore, essential that a narrow sample "plug" be presented at the column during injection in order to provide gas chromatography evaluation in a small period of time. It is for this reason that the dead volume associated with conventional cold trap type gas chromatography systems is a disadvantage. In gas chromatography systems of the type described previously which employ a thermal focusing chamber or cold trap, it must be understood that the entire length of the cold trap sample tube cannot be maintained ideally at a uniform constant temperature, either during the collection or injection modes. In fact, a temperature gradient exists at the inlet and outlet ends of the cold trap capillary tube. Because during the collection mode of operation, the analyte condenses near the inlet end of the capillary tube (in terms of the direction of flow of carrier gas during injection), it is necessary to insure that region is sufficiently heated to vaporize all of the components of interest of the mixture during the injection step. This requirement leads to some portions of the cold trap sample tube being heated to a significantly higher temperature than is necessary to vaporize the sample collected at the inlet end of the sample tube. Furthermore, the analyte is exposed to the excessive temperatures for the length of time necessary to conduct them entirely through the focussing chamber. These excessive temperatures and the significant "residence time" in the sample tube have been related to decomposition of analyte components. Accordingly, instead of components in their natural state being ejected from the column, these components become fragmented into parts of the initial molecule. Such decomposition of the sample significantly complicates analysis and can render the generated chromatogram of little value in certain types of evaluation. An improved cold trap addressing many of the above described disadvantages disclosed in U.S. Pat. No. 5,141,534 which is a continuation-in-part of U.S. patent application Ser. No. 590,174, now U.S. Pat. No. 5,096,471, both herein incorporated by reference.
In a reverse flow cold trapping apparatus, during the trapping mode of operation, the more volatile (lower boiling point) compounds have a tendency to completely traverse the cold trap before the less volatile (higher boiling point) compounds have entered the cold trap. The high rate of traversal of the low boiling point compounds makes it difficult to cryofocus (or cold trap) the lower boiling point and the higher boiling point compounds substantially simultaneously. This generally results in a loss of lower boiling point compounds when attempting to cryofocus and perform detection on the higher boiling point compounds as well. Consequently, it would be extremely beneficial if it were possible to develop a cold trap apparatus which decreases the rate of traversal through the cold trap of the lower boiling point compounds so that the higher boiling point compounds may also be cryofocused substantially simultaneously.
The first described embodiments of a gas chromatography system in accordance with the present invention improves over present devices with respect to each of the previously described areas. In a first embodiment, a cold trap is provided which has a capillary tube for trapping higher boiling point compounds in a first portion of the trap and a porous layer open tubular (PLOT) column to trap the lower boiling point compounds. One end of the capillary tube is positioned in proximity to an analytical column, and sample gas originating from a sample gas source is introduced into the cold trap at this end during collection mode. The other end of the capillary tube interconnects with a first end of the PLOT column, the second end of which interconnects to a vacuum source. The cold trap includes a temperature control source which maintains the cold trap at a low temperature during the collection mode and increases the temperature of the cold trap to vaporize the trapped sample during injection mode. The flow of sample is determined by the status of flow valves in cooperation with interconnected carrier gas and vacuum sources.
Such a cold trap as described above enables the trapping of both higher and lower boiling point compounds during substantially the same time period. Previously, such trapping was not possible because the lower boiling point compounds traversed the capillary tube before the higher boiling point compounds could be trapped. Conversely, if a PLOT column only was used in the cold trap, the lower boiling point compounds could be trapped, but the higher boiling point compounds are slow to elute during injection, backflushing and potentially contaminated the PLOT column.
In a second embodiment, the above described system interconnects to an analytical column to which is applied a source of carrier gas in proximity to its midpoint. The analytical column includes a precolumn portion between the cold trap and the carrier gas inlet and a postcolumn portion between the carrier gas inlet and a flame ionization detector. The two analytical portions enable backflushing the precolumn during application the carrier gas to the analytical column while simultaneously having the applied carrier gas sustain the flow of analyte in the postcolumn, thereby continuing analysis of higher boiling point compounds. The configuration of the second embodiment offers the above described advantages of the first embodiment and further decreases cycle time because backflushing and analysis can occur simultaneously.
Both the embodiments of this invention provide the additional benefit that the inlet system is continually flushed with carrier gas when collection is not taking place, thus further reducing the likelihood of the memory affect discussed previously.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.